Graft polymers of polymers containing vinyl ester and/or vinyl alcohol units and ethylenically unsaturated compounds, their preparation and their use

ABSTRACT

Graft polymers are obtainable by free radical polymerization of 
     (A) monomers or monomer mixtures comprising 
     (a) from 1 to 100% by weight of N-vinylcarboxamides of the formula ##STR1##  where R 1  and R 2  are each H or C 1  -C 6  -alkyl, (b) from 0 to 99% by weight of other monoethylenically unsaturated monomers copolymerizable with the monomers (a) and 
     (c) from 0 to 5% by weight of monomers having at least two ethylenically unsaturated, nonconjugated double bonds in the molecule, 
     in the presence of 
     (B) polymers which contain units of vinyl esters of saturated C 1  -C 4  -carboxylic acids and/or vinyl alcohol units, or of mixtures which contain these polymers and polymers having at least 3 alkylene oxide units or polytetrahydrofuran, 
     in a weight ratio (A):(B) of from 95:5 to 10:90 and, if required, subsequent elimination of the group ##STR2## from the polymerized monomers (I) of the graft polymer with formation of units of the formula ##STR3## and the graft polymers described above are used in the production of paper, board and cardboard as dry and wet strength agents, as retention and drainage aids and as promoters in diketene sizing and as starch cationization agents, as dispersants for pigments and as creping assistants in the production of tissue papers.

Graft polymers of polymers containing vinyl ester and/or vinyl alcoholunits and ethylenically unsaturated compounds, their preparation andtheir use.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to graft polymers of polymers containingvinyl ester and/or vinyl alcohol units and open-chainN-vinylcarboxamides, processes for their preparation and their use inthe production of paper, board and cardboard, as dispersants forpigments and as starch cationization agents.

2. Description of the Background

U.S. Pat. No. 4,880,497 and U.S. Pat. No. 4,978,427 each disclose theproduction of paper having high dry strength and wet strength, ahydrolyzed copolymer obtainable by copolymerization of N-vinylformamideand ethylenically unsaturated monomers, such as vinyl acetate, vinylpropionate or an alkyl vinyl ether, and hydrolysis of from 30 to 100 mol% of the formyl groups of the copolymer with formation of amino groupsbeing used as a strength agent either on the surface of the paper or inthe paper stock prior to sheet formation. The hydrolyzed copolymers areused in amounts of from 0.1 to 5% by weight, based on dry fibers.

EP-A-0 363 319 discloses graft polymers which are obtainable by freeradical polymerization of unsubstituted or N-substituted acrylamide ormethacrylamide and N-vinyl-substituted amides or vinyl esters of asaturated aliphatic monocarboxylic acid in the presence of adducts ofalkylene oxides with a trihydric or polyhydric aliphatic alcohol of 3 to10 carbon atoms. The graft polymers are used for coloring cellulosicfibers with substantive dyes or reactive dyes as colorants.

U.S. Pat. No. 5,334,287 discloses graft polymers which are obtainable byfree radical polymerization of N-vinylcarboxamides, preferablyN-vinylformamide, and, if required, other monomers in the presence ofmonosaccharides, oligosaccharides, polysaccharides or the derivativesthereof in each case and, if required, hydrolysis of the polymerizedN-vinylcarboxamido group with formation of vinylamine units. The graftpolymers are used as dry and wet strength agents in the production ofpaper, board and cardboard.

The earlier non-prior-published German Application 19515943.8 relates tograft polymers which are obtainable by free radical polymerization of

(A) monomers or monomer mixtures comprising

(a) from 10 to 100% by weight of N-vinylcarboxamides of the formula##STR4## where R¹ and R² are each H or C₁ -C₆ -alkyl, (b) from 0 to 90%by weight of other monoethylenically unsaturated monomerscopolymerizable with the monomers (a) and

(c) from 0 to 5% by weight of monomers having at least two ethylenicallyunsaturated, nonconjugated double bonds in the molecule,

in the presence of

(B) polymers which contain at least 3 units of a C₂ -C₄ -alkylene oxide,and/or polytetrahydrofuran in a weight ratio (A):(B) of from 95:5 to10:90, and subsequent elimination of some or all of the groups ##STR5##from the polymerized monomers (a) of the graft polymer with formation ofunits of the formula ##STR6## processes for the preparation of the graftpolymers and the use of the graft polymers in papermaking as dry and wetstrength agents, as fixing agents for interfering substances and dyes,as retention and drainage aids and as promoters in diketene sizing. Thegraft polymers are furthermore used as starch cationization agents andas dispersants for pigments.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide novel substances. Itis a further object of the present invention to provide processassistants for the production of paper, board and cardboard.

We have found that this object is achieved, according to the invention,by graft polymers of polymers containing vinyl ester and/or vinylalcohol units and ethylenically unsaturated compounds, wherein the graftpolymers are obtainable by free radical polymerization of

(A) monomers or monomer mixtures comprising

(a) from 1 to 100% by weight of N-vinylcarboxamides of the formula##STR7## where R¹ and R² are each H or C₁ -C₆ -alkyl, (b) from 0 to 99%by weight of other monoethylenically unsaturated monomerscopolymerizable with the monomers (a) and

(c) from 0 to 5% by weight of monomers having at least two ethylenicallyunsaturated, nonconjugated double bonds in the molecule,

in the presence of

(B) polymers which contain units of vinyl esters of saturated C₁ -C₄-carboxylic acids and/or vinyl alcohol units, or of mixtures whichcontain these polymers and polymers having at least 3 alkylene oxideunits or polytetrahydrofuran

in a weight ratio (A):(B) of from 95:5 to 10:90 and, if required,subsequent elimination of the group ##STR8## from the polymerizedmonomers (a) of the graft polymer with formation of units of the formula##STR9## The present invention furthermore relates to a process for thepreparation of graft polymers from polymers containing vinyl esterand/or vinyl alcohol units and ethylenically unsaturated compounds,wherein

(A) monomers or monomer mixtures comprising

(a) from 1 to 100% by weight of N-vinylcarboxamides of the formula##STR10## where R¹ and R² are each H or C₁ -C₆ -alkyl, (b) from 0 to 90%by weight of other carboxyl-free monoethylenically unsaturated monomerscopolymerizable with the monomers (a) and

(c) from 0 to 5% by weight of monomers having at least two ethylenicallyunsaturated, nonconjugated double bonds in the molecule

are subjected to free radical polymerization in the presence of

(B) polymers which contain units of vinyl esters of saturated C₁ -C₄-carboxylic acids and/or vinyl alcohol units, or of mixtures whichcontain these polymers and polymers having at least 3 alkylene oxideunits or polytetrahydrofuran,

in a weight ratio (A):(B) of from 95:5 to 10:90, and, if required, someor all of the groups ##STR11## are then eliminated from the polymerizedmonomers (a) of the graft polymer with formation of units of the formula##STR12## The present invention also relates to the use of the graftpolymers described above in the production of paper, board and cardboardas dry and wet strength agents, as retention and drainage aids and aspromoters in diketene sizing and as starch cationization agents, asdispersants for pigments and as creping assistants in the production oftissue paper.

DETAILED DESCRIPTION OF THE INVENTION

A preferably used monomer (A) is N-vinylformamide. In a further processstep, from 2 to 100, preferably from 30 to 95, % of the formyl groups ofthe polymerized N-vinylformamide are eliminated from the graft polymersthus obtainable, with formation of units of the formula ##STR13##Preferably used monomer mixtures consist of from 1 to 99% by weight ofN-vinylformamide and from 99 to 1% by weight of vinyl formate, vinylacetate, vinyl propionate, acrylonitrile, N-vinylpyrrolidone,N-vinylcaprolactam, acrylic acid or mixtures of the stated monomers.From 1 to 100, preferably from 30 to 95, % of the formyl groups of thepolymerized N-vinylformamide are eliminated from the graft polymers thusobtainable. Depending on the hydrolysis conditions, it is also possiblechemically to modify the polymerized comonomers, for example vinylalcohol units are formed from the polymerized vinyl esters.

Suitable monomers of group (a) for the preparation of the graft polymersare N-vinylcarboxamides of the formula ##STR14## where R¹ and R² may beidentical or different and are each hydrogen or C₁ -C₆ -alkyl. Examplesof suitable monomers are N-vinylformamide (R¹ ═R² ═H in the formula I),N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide,N-vinyl-N-ethylacetamide, N-vinyl-N-methylpropionamide andN-vinylpropionamide. For the preparation of the graft polymers, thestated monomers may be used either alone or as a mixture with oneanother. N-vinylformamide is preferably used from this group ofmonomers.

The abovementioned N-vinylcarboxamides can, if required, be used, in thegraft polymerization, with other monoethylenically unsaturated monomerscopolymerizable therewith. Examples of suitable monomers of group (b)are vinyl esters of saturated carboxylic acids of 1 to 6 carbon atoms,eg. vinyl formate, vinyl acetate, vinyl propionate and vinyl butyrate.Esters, amides and nitriles of monoethylenically unsaturated C₃ -C₆-carboxylic acids are also suitable. Suitable amides are, for example,acrylamide, methacrylamide and N-alkylmonoamides and N-alkyldiamides,where the alkyl radical in each case is of 1 to 6 carbon atoms, eg.N-methylacrylamide, N,N-dimethylacrylamide, N-methylmethacrylamide,N-ethylacrylamide, N-propylacrylamide and N-tert-butylacrylamide, andthe basic (meth)acrylamides thereof, eg. dimethylaminoethylacrylamide,dimethylaminoethylmethacrylamide, diethylaminoethylacrylamide,diethylaminoethylmethacrylamide, dimethylaminopropylacrylamide,diethylaminopropylacrylamide, dimethylaminopropylmethacrylamide anddiethylaminopropylmethacrylamide. The esters of the monoethylenicallyunsaturated carboxylic acids with C₁ -C₆ -alcohols, eg. methyl acrylate,methyl methacrylate, ethyl acrylate and ethyl methacrylate, or withglycols or polyglycols, where in each case only one OH group of theglycols and polyglycols is esterified with an ethylenically unsaturatedcarboxylic acid, eg. hydroxyethyl acrylate, hydroxyethyl methacrylate,hydroxypropyl acrylates, hydroxybutyl acrylates, hydroxypropylmethacrylates, hydroxybutyl methacrylates and the monoesters of acrylicacid with polyalkylene glycols having a molecular weight of from 1500 to10,000 are also suitable. The esters of ethylenically unsaturatedcarboxylic acids with amino alcohols, eg. dimethylaminoethyl acrylate,dimethylaminoethyl methacrylate, diethylaminoethyl acrylate,diethylaminoethyl methacrylate, dimethylaminopropyl acrylate,dimethylaminopropyl methacrylate, diethylaminopropyl acrylate,diethylaminopropyl methacrylate, dimethylaminobutyl acrylate,diethylaminobutyl acrylate, dimethylaminopentyl acrylate,dimethylaminoneopentyl methacrylate and dimethylaminohexyl acrylate, arefurthermore suitable. The basic acrylates and acrylamides are used inthe form of the free bases, in the form of the salts with mineral acids,eg. hydrochloric acid or sulfuric acid and nitric acid, or inquaternized form. Examples of suitable quaternizing agents are dimethylsulfate, methyl chloride, ethyl chloride, benzyl chloride and diethylsulfate. Monoethylenically unsaturated mono- and dicarboxylic acids oranhydrides of 3 to 6 carbon atoms, eg. acrylic acid, methacrylic acid,crotonic acid, maleic acid or anhydride, fumaric acid, itaconic acid oranhydride and citraconic acid or anhydride, are also suitable.

Other suitable monomers of group (b) are N-vinylpyrrolidone,N-vinylcaprolactam, acrylonitrile, methacrylonitrile, N-vinylimidazoleand substituted N-vinylimidazoles, eg. N-vinyl-2-methylimidazole,N-vinyl-4-methylimidazole, N-vinyl-5-methylimidazole andN-vinyl-2-ethylimidazole, and N-vinylimidazolines, such asN-vinylimidazoline, N-vinyl-2-methylimidazoline andN-vinyl-2-ethylimidazoline. N-vinylimidazoles and N-vinylimidazolinesare used not only in the form of the free bases but also in a formneutralized with mineral acids or in quaternized form, quaternizationbeing effected preferably with dimethyl sulfate, diethyl sulfate, benzylchloride, methyl chloride or ethyl chloride.

Other suitable monomers (b) are sulfo-containing monomers, for examplevinylsulfonic acid, allylsulfonic acid, methallylsulfonic acid,styrenesulfonic acid, 3-sulfopropyl acrylate, 3-sulfopropyl methacryateand 2-acrylamido-2-methylpropanesulfonic acid. The compounds having acidgroups can be used in the graft polymerization in the form of the freeacids or of the ammonium, alkali metal and alkaline earth metal salts.Of the monomers (b), vinyl formate, vinyl acetate, vinyl propionate,acrylonitrile, N-vinylpyrrolidone, N-vinylcaprolactam and acrylic acidare preferred.

In the preparation of the graft polymers, monomer mixtures comprisingfrom 1 to 100% by weight of at least one monomer of group (a) and from 0to 99% by weight of at least one monomer of group (b) are used.

The graft copolymers can be modified by copolymerizing the monomers (a)or monomer mixtures comprising (a) and (b) with up to 5% by weight of amonomer (c) having at least two ethylenically unsaturated, nonconjugateddouble bonds in the molecule, in the presence of the compounds (B). Thecompounds (c) are usually used as crosslinking agents incopolymerizations. They may be added to the monomer mixtures of (a) and,if required, (b) used for the copolymerization. Where they are employed,the preferably used amount is from 0.05 to 2% by weight. The presence ofthe monomers of group (c) during the copolymerization results in anincrease in the molecular weights of the copolymers. Suitable compoundsof this type are, for example, methylenebisacrylamide, esters of acrylicacid and methacrylic acid with polyhydric alcohols, eg. glycoldiacrylate, glyceryl triacrylate, glycol dimethacrylate and glyceryltrimethacrylate, and polyols which are at least diesterified withacrylic or methacrylic acid, such as pentaerythritol and glucose. Othersuitable crosslinking agents are divinylbenzene, divinyldioxane,pentaerythrityl triallyl ether and pentaallylsucrose. Water-solublemonomers, such as glycol diacrylate or glycol diacrylates ofpolyethylene glycols having a molecular weight of up to 3000, arepreferably used from this group of compounds.

The polymerization of the monomers (a) and that of the monomers (a) and(b) and, if required, in each case additionally (c) is carried outaccording to the invention in the presence of (B) polymers which containunits of vinyl esters of saturated C₁ -C₄ -carboxylic acids and/or vinylalcohol units. Such polymers are known. They are prepared, for example,by polymerization of vinyl esters of saturated C₁ -C₄ -carboxylic acids,such as vinyl formate, vinyl acetate, vinyl propionate, vinyln-butyrate, vinyl isobutyrate or mixtures of the stated vinyl esters inthe presence of compounds which form free radicals. Polymers containingvinyl alcohol units are formed therefrom by partial or completehydrolysis of the polymers thus obtainable, for example by the action ofaqueous sodium hydroxide solution or potassium hydroxide solution on thepolymers.

Other suitable grafting bases (B) in addition to the abovementionedpolymers are copolymers of the vinyl esters of saturated C₁ -C₄-carboxylic acids with other, monoethylenically unsaturated monomerswhich are copolymerizable with the vinyl esters. These copolymerspreferably contain at least 20% by weight of at least one vinyl ester aspolymerized units. Examples of suitable comonomers areN-vinylpyrrolidone, N-vinylcaprolactam, maleic anhydride, maleic acid,acrylic acid, methacrylic acid, N-vinylformamide, styrene, vinyl butylether, vinyl ethyl ether, vinyl chloride, butyl acrylate, ethylhexylacrylate, acrylonitrile, N-vinylcarbazole, isobutene and/or methylmethacrylate. In the case of the copolymerization of the vinyl esters,it is also possible to use mixtures of two or more comonomers; forexample, vinyl acetate can be copolymerized with N-vinylpyrrolidone andN-vinylformamide. Suitable components (B) are also polymer blends whichcontain

(1) the polymers described above and containing vinyl ester and/or vinylalcohol units and

(2) polymers having at least 3 alkylene oxide units orpolytetrahydrofuran.

Polymers containing alkylene oxide units and polytetrahydrofurans areknown. Of particular interest are the homo- and copolymers of C₂ -C₄-alkylene oxides. They are prepared, for example, by homo- orcopolymerization of ethylene oxide, propylene oxide, n-butylene oxideand/or isobutylene oxide. The copolymers can be either randomcopolymers, if mixtures of at least 2 alkylene oxides are polymerized,or block copolymers, if an alkylene oxide, for example ethylene oxide,is first polymerized and another alkylene oxide, eg. propylene oxide, isthen polymerized. The block copolymers may be assigned, for example, tothe AB, ABA or BAB type, where A is, for example, a polyethylene oxideblock and B is a block comprising polypropylene oxide. These copolymerscan, if required, also contain n-butylene oxide and/or isobutylene oxideas polymerized units. The polyethylene oxides contain at least 3alkylene oxide units in the molecule. The polyalkylene oxides maycontain, for example, up to 50,000 alkylene oxide units in the molecule.Preferred polyalkylene oxides are those which have from 3 to 1000alkylene oxide units in the molecule. The polytetrahydrofurans contain,for example, from 3 to 200, preferably from 3 to 100, tetramethyleneoxide units.

Preferably used compounds are homopolymers or block copolymers ofethylene oxide and propylene oxide and random copolymers of ethyleneoxide and propylene oxide, which are obtainable by copolymerizing amixed gas comprising ethylene oxide and propylene oxide. For thepurposes of the present invention, polymers containing alkylene oxideunits are also understood as meaning adducts of C₂ -C₄ -alkylene oxideswith alcohols, phenols, carboxylic acids and amines.

Alcohols which are suitable for the reaction with the alkylene oxidesare of, for example, 1 to 30 carbon atoms, eg. methanol, ethanol,n-propanol, isopropanol, n-butanol, n-octanol, 2-ethylhexanol, decanol,dodecanol, palmityl alcohol, cetyl alcohol and stearyl alcohol. Ofparticular industrial interest are the alcohols obtainable by the oxoprocess, for example C₁₀ -alcohols, C₁₃ oxo alcohols or naturalalcohols, such as C₁₀ /C₁₈ -tallow fatty alcohols.

In addition to the stated monohydric alcohols, it is of course alsopossible to use dihydric and polyhydric alcohols for initiating thepolymerization of the alkylene oxides, eg. glycol, glycerol, erythritol,pentaerythritol and sorbitol. The alcohols are reacted with at least oneC₂ -C₄ -alkylene oxide in a molar ratio of from 1:3 to 1:200.

Further suitable polymers containing alkylene oxide units are reactionproducts of fatty acids with alkylene oxides. Particuarly suitable fattyacids are those which contain 8 to 10 carbon atoms in the molecule, forexample lauric acid, myristic acid, stearic acid, palmitic acid, coconutfatty acid, tallow fatty acid and oleic acid.

For the purposes of the present invention, polymers containing ethyleneoxide units are also the adducts of C₂ -C₄ -alkylene oxides with C₁ -C₁₂-alkylphenols, such as n-decylphenol, n-octylphenol, isobutylphenol andmethylphenol. Other suitable components (B) for the preparation of thegraft polymers are the adducts of C₂ -C₄ -alkylene oxides with secondaryC₂ -C₃₀ -amines, such as di-n-butylamine, di-n-octylamine, dimethylamineand distearylamine. The molar ratio of amine to at least one alkyleneoxide is from 1:3 to 1:200, preferably from 1:3 to 1:100. In the case ofthe adducts of alkylene oxides with alcohols, phenols, acids or amines,the alkylene oxides may be subjected in the form of a mixed gas to theaddition reaction with the abovementioned compounds, or the reaction iscarried out first with ethylene oxide and then with propylene oxide. Itis also possible to subject first propylene oxide and then ethyleneoxide to the addition reaction with the stated compounds. Apart fromethylene oxide and propylene oxide, it is also possible, if required, tosubject isobutylene oxide and/or n-butylene oxide to the additionreaction. Block copolymers are formed in the successive addition of thealkylene oxides. In some cases, it may also be advantageous to block thefree OH groups of the alkoxylation products with a terminal group.Blocking with terminal groups can be effected, for example, by means ofan alkyl radical with formation of an ether group. For example, thealkoxylation products can be reacted with alkylating agents, such asdimethyl sulfate. The terminal OH groups can, if required, also beesterified by reaction with carboxylic acids, eg. acetic acid or stearicacid.

Preferably used grafting bases (B) are polyvinyl formate, polyvinylacetate and polymers which are obtainable therefrom by hydrolysis and,in the case of partial hydrolysis, have vinyl alcohol units in additionto unchanged vinyl ester units. In the case of complete hydrolysis ofthe polyvinyl esters, polyvinyl alcohol is obtained. Particularlypreferred grafting bases are those hydrolyzed polyvinyl formates and/orpolyvinyl acetates which contain at least 50 mol % of vinyl alcoholunits.

The molecular weight of the polymers which are suitable as component (B)is preferably from 1000 to 1 million. The polymers preferably used asgrafting base are water-soluble or water-dispersible.

For the preparation of the graft polymers, the monomers of component(A), ie. (a), mixtures of (a) and (b) and, if required, in each caseadditionally (c), are subjected to free radical polymerization in thepresence of polymers of component (B). In some cases, it may beadvantageous with regard to the action of the resulting polymer if twoor more of the compounds stated under (B) are used. The graftpolymerization can be carried out in the presence or absence of inertsolvents or inert diluents. Since the polymerization in the absence ofinert solvents or diluents generally leads to nonuniform polymers, thepolymerization in an inert solvent or diluent is preferred. Examples ofsuitable inert diluents are those in which the compounds stated under(B) can be suspended and which dissolve the monomers (a). In thesecases, the polymers are present in suspended form after thecopolymerization and can readily be isolated in solid form byfiltration. Suitable inert diluents are, for example, toluene, m- andp-xylene and isomer mixtures, ethylbenzene, aliphatic hydrocarbons, suchas pentane, hexane, heptane, octane, nonane, dodecane, cyclohexane,cyclooctane and methylcyclohexane, and mixtures of the statedhydrocarbons or gasoline fractions which contain no polymerizablemonomers. Chlorohydrocarbons, such as chloroform, carbon tetrachloride,hexachloroethane, dichloroethane and tetrachloroethane, are alsosuitable. In the procedure described above, in which the compounds ofcomponent (B) are suspended in an inert diluent, anhydrous compounds ofcomponent (B) are preferably used.

A preferred method for the preparation of the polymers is solutionpolymerization, the compounds of component (B), the monomers (A) and thepolymer formed being present in at least dispersed form but preferablyin dissolved form. For example, inert solvents, such as methanol,ethanol, isopropanol, n-propanol, n-butanol, sec-butanol,tetrahydrofuran, dioxane, water and mixtures of the stated inertsolvents, are suitable for the solution polymerization. Thepolymerization can be carried out continuously or batchwise.

The graft polymers are generally prepared in the presence of freeradical initiators.

Preferred free radical initiators are all those compounds which have ahalf life of less than 3 hours at the polymerization temperature chosenin each case. If the polymerization is first initiated at a lowertemperature and is completed at a higher temperature, it is advantageousto use at least two initiators which decompose at differenttemperatures, ie. first to use an initiator which decomposes at a lowertemperature for initiating the polymerization and then to complete themain polymerization with an initiator which decomposes at a highertemperature. Water-soluble and water-insoluble initiators or mixtures ofwater-soluble and water-insoluble initiators may be used. Thewater-insoluble initiators are then soluble in the organic phase. Forthe temperature ranges stated below, for example, the initiators statedfor said ranges can be used.

Temperature: from 40 to 60° C.

Acetylcyclohexanesulfonyl peroxide, diacetyl peroxydicarbonate,dicyclohexyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate,tert-butyl perneodecanoate,2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile),2,2'-azobis(2-methyl-N-phenylpropionamidine) dihydrochloride,2,2'-azobis(2-methylpropionamidine) dihydrochloride.

Temperature: from 60 to 80° C.

tert-Butyl perpivalate, dioctanoyl peroxide, dilauroyl peroxide,2,2'-azobis(2,4-dimethylvaleronitrile).

Temperature: from 80 to 100° C.

Dibenzoyl peroxide, tert-butyl per-2-ethylhexanoate, tert-butylpermaleate, 2,2'-azobisisobutyronitrile, dimethyl2,2'-azobisisobutyrate, sodium persulfate, potassium persulfate,ammonium persulfate.

Temperature: from 100 to 120° C.

Bis(tert-butylperoxy)cyclohexane, tert-butyl peroxyisopropylcarbonate,tert-butyl peracetate, hydrogen peroxide.

Temperature: from 120 to 140° C.

2,2-Bis(tert-butylperoxy)butane, dicumyl peroxide, di-tert-amylperoxide, di-tert-butyl peroxide.

Temperature: >140° C.

Menthane hydroperoxide, pinane hydroperoxide, cumyl hydroperoxide andtert-butyl hydroperoxide.

If salts or complexes of heavy metals, for exmaple copper, cobalt,manganese, iron, vanadium, nickel and chromium salts, or organiccompounds such as benzoin, dimethylaniline or ascorbic acid, are used inaddition to the stated initiators, the half-lives of the stated freeradical initiators can be reduced. For example, tert-butyl hydroperoxidecan be activated with the addition of 5 ppm of copper(II)acetylacetonate so that polymerization can be effected at as low as 100°C. The reducing component of redox catalysts may also be, for example,compounds such as sodium sulfite, sodium bisulfite, sodium formaldehydesulfoxylate and hydrazine. From 0.01 to 20, preferably from 0.05 to 10,% by weight, based on the monomers used in the polymerization, of apolymerization initiator or of a mixture of a plurality ofpolymerization initiators are used. From 0.01 to 15% of the reducingcompounds are added as redox components. Heavy metals are used in anamount of from 0.1 to 100 ppm, preferably from 0.5 to 10 ppm. It isoften advantageous to use a combination of peroxide, reducing agent andheavy metal as the redox catalyst.

The graft polymerization of the essential monomers (a) and, if required,(b) and of the monomers (c) which may be present if required can also becarried out by the action of ultraviolet radiation, in the presence orabsence of UV initiators. The photoinitiators or sensitizers usuallyused for this purpose are employed for the polymerization under theaction of UV radiation. These are, for example, compounds such asbenzoin and benzoin ethers, α-methylbenzoin or α-phenylbenzoin. Tripletsensitizers, such as benzil diketals, may also be used. The UV radiationsources used are, for example, high-energy UV lamps, such as carbon arclamps, mercury vapor lamps or xenon lamps, as well as low-UV lightsources, such as fluorescent tubes having a large blue component.

In order to prepare polymers having a low K value, the polymerization isadvantageously carried out in the presence of regulators. Examples ofsuitable regulators are organic compounds containing sulfur in bondedform. These include, for example, mercapto compounds, such asmercaptoethanol, mercaptopropanol, mercaptobutanol, mercaptoacetic acid,mercaptopropionic acid, butyl mercaptan and dodecyl mercaptan. Othersuitable regulators are allyl compounds, such as allyl alcohol,aldehydes, such as formaldehyde, acetaldehyde, propionaldehyde,n-butyraldehyde and isobutyraldehyde, formic acid, ammonium formate,propionic acid, hydrazine sulfate and butenols. If the polymerization iscarried out in the presence of regulators, from 0.05 to 20% by weight,based on the monomers used in the polymerization, of said regulators arerequired.

The graft polymerization of components (A) and (B) is usually carriedout in an inert atmosphere in the absence of atmospheric oxygen. Duringthe polymerization, thorough mixing of the reactants is generallyensured. In the case of relatively small batches in which reliableremoval of the heat of polymerization is ensured, the reactants, whichare preferably present in an inert diluent, can be copolymerizedbatchwise by heating the reaction mixture to the polymerizationtemperature and then allowing the reaction to take place. Thesetemperatures are from 40 to 180° C. To permit better control of thecourse of the polymerization reaction, the monomers (A) are added, atthe desired polymerization temperature, continuously or batchwise to thepolymerizing mixture at a rate such that the polymerization is readilycontrollable in the desired temperature range. In a preferred method foradding the monomers of component (A), the compounds of component (B) orat least a part of the compounds of component (B) are or is initiallytaken in the polymerization reactor and heated to the desiredpolymerization temperature therein while stirring. As soon as thistemperature is reached, the monomers (a) and, if required, (b) and, ifrequired, (c) and the initiator and, if required, a regulator are addedover a period of from about 1 to 10, preferably from 2 to 8, hours. Sucha procedure is used, for example, in the polymerization of components(A) and (B) in an inert diluent in which component (B) is suspended, andalso in the polymerization carried out in solution.

The novel graft polymers are preferably prepared by suspension orsolution polymerization of components (A) and (B) in an aqueous medium,solution polymerization in water being particularly preferred. In thesolution polymerization in an aqueous medium, for example, at least apart of the compounds of component (B) is initially taken in the aqueousmedium and the monomers (a) and, if required, (b) and, if required, themonomers (c) are added continuously or batchwise to the polymerizingreaction mixture. In order to avoid hydrolysis of the monomericN-vinylcarboxamides during the polymerization in aqueous solution, thepolymerization is preferably carried out at a pH of from 4 to 9, inparticular from 5 to 8. In many cases, it is advisable additionally towork in the presence of buffers, for example to add primary or secondarysodium phosphate to the aqueous phase. When monomers (b) containing acidgroups are used, they are preferably employed in the form of the salts.

The weight ratio (A):(B) is from 95:5 to 10:90. 90).

In the graft polymerization, from 1 to 5 parts by weight of component(A) are preferably used per part by weight of component (B).

In the graft polymerization, the temperatures are usually from 40 to180° C., preferably from 50 to 150° C., in particular from 60 to 110° C.As soon as the temperature in the graft polymerization is above theboiling points of the inert diluents or solvents or of the monomers, thepolymerization is carried out under superatmospheric pressure. Theconcentration of components (A) and (B) in the graft polymerization inthe presence of inert solvents or inert diluents is from 10 to 80,preferably from 20 to 70, % by weight. The preparation of the graftpolymers can be carried out in the conventional polymerizationapparatuses. For example, stirred kettles which are equipped with ananchor stirrer, paddle stirrer, impeller or multistage impulsecountercurrent agitator are used for this purpose. Particularly in thepolymerization in the absence of diluents, it may be advantageous tocarry out the polymerization in a kneader. It may also be necessary toeffect polymerization in a kneader if high concentrations are used.

Graft polymers which, where they are soluble in water, have K values offrom 8 to 250, preferably from 10 to 150 (measured in 1% strengthaqueous solutions of the copolymers at pH 7 and 25° C.) are obtained.The graft polymers which can be prepared by the abovementioned processesare colorless to brownish products. In the case of polymerization in anaqueous medium, they are obtained in the form of dispersions or polymersolutions. Depending on the particular composition of the graftpolymers, low-viscosity to pasty aqueous solutions or aqueousdispersions are obtained.

The preparation of the graft polymers may be followed by a secondprocess stage in which hydrolysis is carried out under the action ofacids, bases or enzymes. In the hydrolysis, the group ##STR15## iseliminated from the monomers of the formula I which are present aspolymerized units in the graft polymer. The resulting hydroyzed graftpolymers contain, in the grafted-on side chain, units of the formulae##STR16## where R¹ and R² have the meanings stated in the formula I. Inthe hydrolysis of the graft polymers, some or all of any vinyl esterunits contained in the grafting base (B) may be hydrolyzed to vinylalcohol units. In a hydrolysis of the graft polymers in the presence ofacids, the units of the formula III are present in the form of thesalts, while in the hydrolysis in the presence of bases the free aminogroups are present, depending on the pH.

Depending on the reaction conditions in the hydrolysis, ie. the amountof acid or base, based on the polymer to be hydrolyzed, and on thereaction temperature during the hydrolysis, either partial or completehydrolysis of the units of the formula Ia results. The hydrolysis of thegraft polymers is continued until from 1 to 100%, preferably from 30 to95%, of the monomer units of the formula (II) which are contained in thegraft polymers have been hydrolyzed. For the hydrolysis, at least oneacid or base is added to the graft polymers prepared in the firstprocess stage. Suitable acids are, for example, mineral acids, such ashydrogen halide, (in gaseous form or in aqueous solution), sulfuricacid, nitric acid or phosphoric acid (ortho-, meta- or polyphosphoricacid), and organic acids, for example C₁ -C₅ -carboxylic acids, such asformic acid, acetic acid and propionic acid, or the aliphatic oraromatic sulfonic acids, such as methanesulfonic acid, benzenesulfonicacid or toluenesulfonic acid. Hydrochloric acid or sulfuric acid ispreferably used for the hydrolysis. In the hydrolysis with acids, the pHis from 0 to 5. For example, from 0.05 to 1.5, preferably from 0.4 to1.2, equivalents of acid are required per formyl group equivalent in thepolymer.

In the hydrolysis with bases, hydroxides of metals of the first andsecond main groups of the Periodic Table may be used; for example,lithium hydroxide, sodium hydroxide, potassium hydroxide, calciumhydroxide, strontium hydroxide and barium hydroxide are suitable.However, it is also possible to use ammonia and alkyl derivatives ofammonia, for example alkylamines or arylamines, eg. triethylamine,monoethanolamine, diethanolamine, triethanolamine, morpholine oraniline. In the hydrolysis with bases, the pH is from 8 to 14. The basescan be used in the solid, liquid or, if required, also gaseous state,diluted or undiluted. A preferably used base for the hydrolysis isammonia, sodium hydroxide solution or potassium hydroxide solution. Thehydrolysis at acidic or alkaline pH is carried out, for example, at from30 to 170° C., preferably from 50 to 120° C. It is complete after fromabout 2 to 8, preferably from 3 to 5, hours. After this reaction time,from 1 to 100% of the polymerized monomers of the formula I have beenhydrolyzed. A procedure in which the bases or acids are added in aqueoussolution for the hydrolysis has proven particularly useful. After thehydrolysis, a neutralization is generally carried out, so that the pH ofthe hydrolyzed polymer solution is from 2 to 8, preferably from 3 to 7.The neutralization is required when it is intended to prevent or delay acontinuation of the hydrolysis of partially hydrolyzed polymers. Thehydrolysis can also be carried out with the aid of enzymes.

Particularly preferred graft polymers are those which have been preparedusing N-vinylformamide or monomer mixtures of

(a) from 1 to 99% by weight of N-vinylformamide and

(b) from 99 to 1% by weight of vinyl formate and/or vinyl acetate

as monomer (A) and polyvinyl alcohols which may be esterified to adegree of upto 25 mol % with saturated C₁ -C₄ -carboxylic acids aspolymer (B) and which are then subjected to hydrolysis in which from 1to 100% of the formyl groups of the grafted-on N-vinylformamide areeliminated from the graft polymer with formation of units of the formula##STR17## and from 2 to 100% of the formate and/or acetate groups of therafted-on vinyl formate and/or vinyl acetate are eliminated withformation of units of the formula ##STR18## In the acidic hydrolysis ofgraft polymers which contain not only N-vinylformamide but alsoacrylonitrile as polymerized units, acrylamide units and/or acrylic acidunits and imide structures of the formula ##STR19## may also be formedfrom the last-mentioned monomer. The proportion of these structures inthe hydrolyzed graft polymer may be from 0 to 60 mol % of the units(VI), depending on the amount of polymerized acrylonitrile and on thereaction conditions. On the other hand, the hydrolysis with bases, inparticular metal hydroxides, leads to substantial formation ofcarboxylate functions.

The K values of the hydrolyzed graft polymers are from 8 to 250,preferably from 10 to 150 (measured in 1% strength aqueous solutions ofthe polymers at pH 7 and 25° C.), where the polymers are soluble inwater.

To prevent or substantially suppress a decrease in the efficiency of thehydrolyzed graft polymers during use, as a result of storage, and toobtain a substantially color-stable polymer solution, antioxidants,reducing agents or aldehyde scavengers may be added during or after thehydrolysis.

Antioxidants, which generally act as free radical scavengers or UVstabilizers, are, for example, secondary aromatic amines, phenol,alkylphenols, thioethers, phosphites or mixtures of compounds of thestated classes. Suitable secondary aromatic amines are, for example,4,4'-bis(tert-butyl)diphenylamine, 4,4'-bis(phenylmethyl)diphenylamineor mixtures thereof. Alkylphenols which are suitable antioxidants are,for example 2,6-dimethyl-4-tert-butylphenol, 2,4,6-trimethylphenol,2,4-di-tert-butyl-6-methylphenol or mixtures thereof. Examples ofsuitable thioethers are dialkyl 3,3'-thiodipropionate,poly-2,3-dimethylphenyl 1,4-disulfide,bis(2-methyl-4-hydroxy-5-tert-butyl) sulfide, dibenzyl sulfide anddialkyl disulfides, eg. dioctadecyl disulfide.

Phosphites which are suitable antioxidants are, for example,trisnonylphenyl phosphite, di(2,4-di-tert-butylphenyl) pentaerythrityldiphosphite and diphenylene decyl phosphite.

Examples of suitable reducing agents are sodium borohydride, sodiumcyanoborohydride and dithionites, such as sodium, potassium or zincdithionite.

Aldehyde scavengers are, for example, NH-containing compounds, such asurea, ethyleneurea, propyleneurea, melamine, guanidine,phenylbiguanidine or mixtures of the stated compounds. Other aldehydescavengers are, for example, alkali metal bisulfites, such as sodium orpotassium bisulfite.

Antioxidants, reducing agents and aldehyde scavengers are each used inamounts of from 0.01 to 20, preferably from 0.1 to 16, % by weight,based on the polymers. These substances may be added before, during orafter the hydrolysis of the amido groups contained in the graftpolymers.

The graft polymers obtained in this manner and containingN-vinylcarboxamide and/or vinylamine units are used in papermaking forincreasing the dry and wet strength of the paper. The novel graftpolymers, which may be hydrolyzed, are preferably used in aqueoussolution and are added to the paper stock before sheet formation, in anamount of from 0.1 to 10% by weight, based on dry fibers. The aqueouspolymer solutions may also be applied to the surface of the paper, theamounts to be used being from 0.1 to 10, preferably from 0.25 to 3, % byweight, based on dry fibers. The aqueous solutions of the polymers areeffective in the case of all known paper, board and cardboard qualities,for example in the production of hygiene, writing, printing andpackaging papers. The papers or boards and cardboards may be producedfrom a large number of fiber materials, for example from sulfite orsulfate pulp (bleached or unbleached), groundwood pulp,chemothermomechanical pump (CTMP), thermomechanical pulp (TMP) orwastepaper or mixtures of the stated fiber types. The pH of the stocksuspension is from 4 to 9, preferably from 6 to 8. The copolymersdescribed above are preferably added in an amount of from 0.25 to 2% byweight, based on dry fibers, to the paper stock suspension before sheetformation and lead to an increase in the dry and wet strength of thepaper.

The graft polymers, which may be hydrolyzed, are also suitable as fixingagents for interfering substances and dyes in the production of paper,board and cardboard. For this intended use, the graft polymers are addeddirectly to the paper stock or may be added to the paper stock in theform of a mixture with the resin size. For example, from 1 to 100,preferably from 5 to 30, parts by weight, based on 100 parts by weightof resin size, of the graft polymers are used.

Graft polymers which have a high molecular weight, for example K valuesof from about 150 to 250, are used as retention aids and drainage aidsin the production of paper, board and cardboard. Usually, from 0.01 to5, preferably from 0.1 to 2, % by weight, based on dry fibers, of graftpolymers are suitable for this intended use.

A further application of the novel graft polymers, in particular ofhydrolyzed graft polymers, is as starch cationization agents. In orderto cationize starch, for example, an aqueous suspension of starch isheated to 80-180° C. in the presence of the graft polymers. Attemperatures above the boiling point of the aqueous reaction mixtures,closed pressure-resistant apparatuses are used. For example, from 0.1 to100, preferably from 1 to 10, % by weight, based on starch, of at leastone graft polymer are used in the starch cationization. All starch typescan be cationized with the novel graft polymers, for example naturalstarches, such as potato, rice, corn and wheat starches, as well asdegraded starches or starch types having amylopectin contents of from atleast 95 to 100%, for example wax corn starches or wax potato starches.Those graft polymers in which the degree of hydrolysis of thepolymerized N-vinylcarboxamides is at least 60% are particularlysuitable for this intended use. The cationized starches thus preparedare used, for example, in papermaking. The result is an increase in thedry and wet strength of the paper and are distinguished by particularlyhigh retention compared with unmodified starches.

The novel graft polymers may also be used as dispersants for pigments.The amounts usually used for this purpose are from about 0.1 to 5,preferably from 0.5 to 2, % by weight, based on the pigments. Suitablepigments are, for example, chalk, clay, talc and titanium dioxide.Highly concentrated aqueous pigment suspensions are prepared for use asfillers in appermaking or for the preparation of paper coating slips.Such pigment suspensions may contain up to 75% by weight of a pigment.

The novel graft polymers are furthermore suitable as promoters in thediketene sizing of paper, board and cardboard. The graft polymers areemulsified together with the diketene in water in the preparation of thediketene emulsion for this purpose. The diketene emulsions contain, forexample, from 0.05 to 5% by weight of a graft polymer. The novel graftpolymers result in rapid formation of the diketene size. The graftpolymers are furthermore suitable as creping assistants in theproduction of tissue papers. They are used for this purpose in amountsof from 0.05 to 5% by weight, based on dry fibers.

In the examples which follow, parts and percentages are by weight.

The K values of the polymers were determined according to H.Fikentscher, Cellulose-Chemie, 13 (1932), 58-64 and 71-74, in 1%strength by weight aqueous solution at 25° C.

The paper sheets are produced in a Rapid-Kothen laboratory sheet former.The dry breaking length was determined according to DIN 53112, sheet 1,and the wet breaking length according to DIN 53112, sheet 2.

EXAMPLE 1

829.5 g of distilled water, 1.27 g of 75% strength phosphoric acid and0.87 g of 50% strength aqueous sodium hydroxide solution and 33 g ofpolyvinyl alcohol containing 2 mol % of acetate groups and having amolecular weight of 27,000 are introduced into a heatable reactorequipped with a stirrer, a reflux condenser, a thermometer, feedapparatuses and nitrogen inlet and outlet apparatuses, and the pH of themixture is brought to 6.5 with phosphoric acid or sodium hydroxidesolution. The reactor content is then heated to 70° C. in a gentlestream of nitrogen (10 l/h), and 134.7 g of N-vinylformamide are meteredin uniformly in the course of 3 hours and a solution of 0.53 g of2,2'-azobis(2-methylpropionamidine) dihydrochloride in 100 g ofdistilled water is metered in uniformly in the course of 4 hours.Heating is then continued for a further 2 hours at 70° C. The slightlycloudy, colorless, viscous solution has a solids content of 16.1% and aK value of 81.

Hydrolysis:

500 g of the graft polymer solution described above are initially takenin a stirred apparatus equipped with a reflux condenser, a thermometerand a dropping funnel. 103 g of 38% strength hydrochloric acid are addeddropwise in the course of 15 minutes with thorough stirring. Thereaction mixture is then heated at 70° C. for 6 hours. The conversion isdetermined by polyelectrolyte titration. After the mixture has cooled toroom temperature, a pH of 3.8 is established by slow dropwise additionof a total of 77.6 g of 50% strength aqueous sodium hydroxide solution.11 g of 30% strength sodium bisulfite solution are added to the solutionand stirring is continued for 10 minutes. The degree of hydrolysis ofthe polymerized N-vinylformamide is 91%, the solids content 21.8%, thepolymer content 12.2% and the K value 80.5.

EXAMPLE 2

831 g of distilled water, 0.96 g of aqueous 75% strength phosphoricacid, 0.66 g of 50% strength aqueous sodium hydroxide solution and 66 gof polyvinyl alcohol containing 2 mol % of acetate groups and having amolecular weight of 27,000 are introduced into a reactor according toExample 1, and the mixture is brought to a pH of 6.5 as in Example 1.The mixture is then heated to 70° C. in a gentle stream of nitrogen (10l/h), and 101 g of N-vinylformamide are metered in uniformly in thecourse of 3 hours and a solution of 0.4 g of2,2'-azobis(2-methylpropionanidine) dihydrochloride in 100 g ofdistilled water is metered in uniformly in the course of 4 hours at 70°C. Heating is then continued for a further 2 hours at 70° C. The cloudy,colorless solution has a solids content of 14.9%. The K value of thegraft polymer is 72.8.

Hydrolysis:

500 g of the graft polymer solution described above are initially takenin a stirred apparatus equipped with a reflux condenser, a thermometerand a dropping funnel. 74.8 g of 38% strength hydrochloric acid areadded dropwise in the course of 10 minutes 30 with thorough stirring.The solution is then heated at 70° C. for 6 hours. The conversion isdetermined by polyelectrolyte titration. After the mixture has cooled toroom temperature, a pH of 3.8 is established by adding 56 g of 50%strength aqueous sodium hydroxide solution a little at a time. 10.6 g of30% strength sodium bisulfite solution are added, and stirring iscontinued for a further 10 minutes. 641.4 g of an aqueous solution of ahydrolyzed graft polymer are obtained. The degree of hydrolysis of thepolymerized N-vinylformamide is 85%. The solution has a solids contentof 20% and a polymer content of 12.5%. The K value of the hydrolyzedgraft polymer is 73.

EXAMPLE 3

Example 2 is repeated, with the exceptions that the amount of polyvinylalcohol is increased to 99 g and 67.3 g of N-vinylformamide are meteredin over 3 hours. The cloudy, colorless solution has a solids content of14.3%. The K value of the graft polymer is 62.9.

Hydrolysis:

As stated in Example 2, 500 g of the graft polymer solution describedabove are hydrolyzed with 47 g of 38% strength hydrochloric acid. 35 gof 50% strength sodium hydroxide solution and 9.7 g of 30% strengthsodium bisulfite solution are then added. The degree of hydrolysis ofthe polymerized N-vinylformamide is 79%. The slightly cloudy solutionhas a solids content of 17.6% and a polymer content of 12.6%. The Kvalue of the hydrolyzed graft polymer corresponds to the K value of theunhydrolyzed graft polymer.

EXAMPLE 4

Example 1 is repeated, with the single exception that, instead of thepolymer used there, a polyvinyl alcohol which contains 2 mol % ofacetate groups and has a molecular weight of 61,000 is now used. Acloudy, colorless solution having a solids content of 15.3% is obtained.The K value of the polymer is 23.1.

Hydrolysis:

As described in Example 1, 500 g of the graft polymer solution describedabove are hydrolyzed with 102 g of 38% strength hydrochloric acid. 69.1g of 50% strength aqueous sodium hydroxide solution and 11 g of 30%strength sodium bisulfite solution are then added. The degree ofhydrolysis of the polymerized N-vinylformamide units is 94%. Thesolution has a solids content of 21.1% and a polymer content of 12.3%.The K value of the hydrolyzed graft polymer is 22.8.

EXAMPLE 5

The procedure is as described in Example 1, except that the graftingbase used is a polyvinyl alcohol which contains 12 mol % of acetategroups and has a molecular weight of 130,000. A cloudy, colorlesssolution having a solids content of 15.3% is obtained. The graft polymerhas a K value of 81.

Hydrolysis:

500 g of the aqueous polymer solution described above are hydrolyzed, asdescribed in Example 1, by adding 102 g of 38% strength hydrochloricacid. After the reaction mixture has cooled, 69.9 g of 50% strengthaqueous sodium hydroxide solution are added for adjusting the pH, and 11g of a 30% strength aqueous sodium bisulfite solution are added. Thesolution thus obtained has a solids content of 21.7% and contains 12.1%of polymer. The degree of hydrolysis of the polymerized N-vinylformamideis 91%. The hydrolyzed graft polymer has virtually the same K value asthe unhydrolyzed polymer.

EXAMPLE 6

395 g of distilled water, 2.62 g of 75% strength aqueous phosphoricacid, 1.8 g of 50% strength aqueous sodium hydroxide solution, 1.44 g ofan aqueous solution of the sodium salt of a molar copolymer of maleicacid and styrene, having a molecular weight of 150,000, and 75.6 g ofthe polyvinyl alcohol described in Example 1 are initially taken in thereactor described in Example 1 and brought to a pH of 6.5. The mixtureis then heated to 65° C. in a gentle stream of nitrogen while stirring,and, at this temperature, 34 g of vinyl acetate are added uniformly inthe course of 2 hours, 81 g of N-vinylformamide are added uniformly andseparately therefrom in the course of 3 hours and a solution of 0.25 gof 2,2'-azobis(2-methylpropionamidine) dihydrochloride is addeduniformly in the course of 4 hours, likewise separately therefrom.During the polymerization, the reaction mixture is highly viscous. It istherefore diluted with 300 g of distilled water. After the addition ofthe initiator, the reaction mixture is stirred for a further 2 hours at65° C., after which a solution of 0.05% of2,2'-azobis(2-methylpropionamidine) dihydrochloride in 1 g of distilledwater is added all at once. The temperature of the reaction mixture isthen increased to 98° C. in the course of 4 hours and the reactionsolution is then cooled. A clear, colorless solution having a solidscontent of 17% is obtained. The polymer has a K value of 76.2.

Hydrolysis:

500 g of the polymer solution described above are hydrolyzed by themethod stated in Example 1, by adding 65.4 g of 38% strengthhydrochloric acid. After the hydrolysis, 45.2 g of a 50% strengthaqueous sodium hydroxide solution and 7.5 g of a 30% strength aqueoussodium bisulfite solution are added. The reaction mixture has a solidscontent of 22% and a polymer content of 15.5%. The degree of hydrolysisof the polymerized N-vinylformamide is 95% and that of the polymerizedvinyl acetate is 35%. The hydrolyzed graft polymer has virtually thesame K value as the graft polymer before the hydrolysis.

EXAMPLE 7

829 g of distilled water, 1.27 g of 75% strength aqueous phosphoricacid, 0.87 g of 50% strength aqueous sodium hydroxide solution and 66 gof polyvinyl alcohol described in Example 1 are initially taken in thereactor described in Example 1, brought to a pH of 6.7 and heated to 70°C. under a gentle stream of nitrogen and while stirring. As soon as thereactor content has reached this temperature, a mixture of 50.5 g ofN-vinylformamide and 49.5 g of acrylonitrile is metered in uniformly inthe course of 2 hours and, separately therefrom, a solution of 0.4 g of2,2'-azobis(2-methylpropionamidine) dihydrochloride in 100 g ofdistilled water is metered in uniformly in the course of 4 hours. Afine, white 13.7% strength polymer suspension is obtained.

Hydrolysis:

500 g of the polymer suspension described above are initially taken inan apparatus equipped with a stirrer, and 42.4 g of 38% strengthhydrochloric acid are added in the course of 10 minutes. The suspensionis stirred for 8 hours at 70° C. Thereafter, the reaction mixture isallowed to cool, the viscous solution is diluted with 140 g of water anda pH of 2.5 is established by adding 34.0 g of 25% strength aqueoussodium hydroxide solution. 11.7 g of a 30% strength aqueous sodiumbisulfite solution are then also added, and the reaction mixture isstirred for a further 15 minutes. It has a solids content of 10.8% and apolymer content of 6.9%. The degree of hydrolysis of the polymerizedN-vinylformamide is 90%. The K value of the hydrolyzed graft polymercorresponds to that of the unhydrolyzed polymer.

EXAMPLE 8

829 g of distilled water, 1.27 g of 75% strength aqueous phosphoricacid, 0.87 g of 50% strength aqueous sodium hydroxide solution and 66 gof the polyvinyl alcohol described in Example 1 are initially taken inthe reactor described in Example 1, brought to a pH of 7.3 and heated to70° C. under a gentle stream of nitrogen and while stirring. As soon asthe reactor content has reached this temperature, 80.8 g ofN-vinylformamide and 19.8 g of N-vinylpyrrolidone are each metered inuniformly in the course of 2 hours and, separately therefrom, a solutionof 0.4 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride in 100 gof distilled water is metered in uniformly in the course of 4 hours. Acloudy, colorless 15.3% strength solution is obtained. The graft polymerhas a K value of 77.6.

Hydrolysis:

As described in Example 1, 500 g of the graft polymer solution describedabove are hydrolyzed with 70.5 g of 33% strength hydrochloric acid,after which 54.8 g of 25% strength sodium hydroxide solution and 10.1 gof 30% strength sodium sulfite solution are added. The pH of the productis 2.5 and the degree of hydrolysis of the polymerized N-vinylformamideunits is 100%.

The solution has a solids content of 17% and a polymer content of 12.7%.The K value of the hydrolyzed graft polymer corresponds to the K valueof the unhydrolyzed polymer.

EXAMPLE 9

729 g of distilled water, 1.27 g of 75% strength aqueous 20 phosphoricacid, 0.87 g of 50% strength aqueous sodium hydroxide solution and 66 gof the polyvinyl alcohol described in Example 1 are initially taken inthe reactor described in Example 1, brought to a pH of 6.5 and heated to70° C. under a gentle stream of nitrogen and while stirring. As soon asthe reactor content has reached this temperature, 70.7 g ofN-vinylformamide and 160 g of an aqueous acrylic acid solution broughtto pH 7.3 with NaOH (29.7 g of acrylic acid) are each metered inuniformly in the course of 2 hours and, separately therefrom, a solutionof 0.4 g of 2,2'-azobis(2-methylpropionamidine) dihydrochloride in 100 gof distilled water is metered in uniformly in the course of 4 hours. Acloudy, colorless 15.5% strength solution is obtained. The graft polymerhas a K value of 77.6.

Hydrolysis:

As described in Example 1, 500 g of the graft polymer solution describedabove are hydrolyzed with 72.5 g of 38% strength hydrochloric acid. 38.7g of 50% strength aqueous sodium hydroxide solution and 12 g of 30%strength sodium bisulfite solution are then added. The pH of thesolution is 2.5 and the degree of hydrolysis of the polymerizedvinylformamide units is 95%. The solution has a solids content of 18%and a polymer content of 12.4%. The K value of the hydrolyzed graftpolymer is 77.

COMPARATIVE EXAMPLE 1

74 g of 38% strength hydrochloric acid (120 mol %, based onN-vinylformamide) are added dropwise to 300 g of a 15.3% strengthaqueous polyvinylformamide solution (K value of the polymer is 85). Themixture is then heated at 70° C. for about 5 hours. The degree ofhydrolysis (>93%) is monitored by polyelectrolyte titration. Aftercooling, the pH of the solution is increased to 3.5 with 50% strengthsodium hydroxide solution (40.6 g). The polymer content of the solutionis 10.9% by weight.

COMPARATIVE EXAMPLE 2

A copolymer of 70% by weight of N-vinylformamide and 30% by weight ofvinyl acetate, having a K value of 85, is prepared according to U.S.Pat. 4,978,427 and is hydrolyzed by adding 110 mol % of a 38% strengthhydrochloric acid per mole of N-vinylformamide contained in the polymer,to such an extent that at least 90% of the polymerized N-vinylformamideand at least 80% of the polymerized vinyl acetate are hydrolyzed.

COMPARATIVE EXAMPLE 3

Commercial neutral wet strength resin based on a urea/formaldehydecondensate

Use Examples

EXAMPLE 10

Sheets having a basis weight of 80 g/m² were produced in a Rapid-Kothensheet former. The paper stock consisted of 50% of bleached hardwoodsulfite pulp and 50% of bleached softwood sulfite pulp having a freenessof 32⁰ SR (Schopper-Riegler) in 0.5% strength aqueous suspension. The pHof the stock suspension was 7.0. The stock suspension was divided into 8equal parts. The substances stated under (b) to (h) were added to 7samples:

(a) The stock suspension contained no further additives.

(b) 1%, based on dry fibers, of a commercial resin according tocomparative product 3 was added to the stock suspension.

(c) 1%, based on dry fibers, of the hydrolyzed polyvinylformamideaccording to Comparative Example 1 was added to the stock suspension.

(d-h) 1%, based on dry fibers, of the hydrolyzed graft polymer preparedaccording to Examples 1 to 5 was added to the stock suspension (Examplesaccording to the invention).

The paper stock suspensions (a) to (h) described above were thenconverted into the sheets (a) to (h) on a Rapid-Kothen sheet former. Thewet breaking length and the dry breaking length of the sheets obtainedwere determined. The results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                                       Dry breaking                                     Graft polymer Wet breaking length [m] length                                Exam- according to        aged       [m]                                        ple 10 Example unaged (5 min at 110° C.) unaged                      ______________________________________                                        a)    none       116        122      3302                                       b) Comparison 3 799 821 3541                                                  c) Comparison 2 808 815 4065                                                  d) 1 774 816 3953                                                             e) 2 701 799 4087                                                             f) 3 781 699 3758                                                             g) 4 821 839 3892                                                             h 5 806 869 3953                                                            ______________________________________                                    

EXAMPLE 11

Sheets having a basis weight of 80 g/m² were produced in a Rapid-Kothensheet former. The paper stock consisted of 50% of bleached hardwoodsulfite pulp and 50% of bleached softwood sulfite pulp having a freenessof 32°SR (Schopper-Riegler) in 0.5% strength aqueous suspension. The pHof the stock suspension was 4.5. The stock suspension was divided into 8equal parts. The substances stated under (b) to (h) were added to 7samples:

(a) The stock suspension contained no further additives.

(b) 1%, based on dry fibers, of the commercial product according tocomparative example 3 was added to the stock suspension.

(c) 1%, based on dry fibers, of the product according to comparativeexample 2 was added to the stock suspension.

(d-h) 1%, based on dry fibers, of the hydrolyzed graft polymer preparedaccording to Examples 1 to 5 was added to the stock suspension (Examplesaccording to the invention).

The paper stock suspensions (a) to (h) described above were convertedinto paper sheets, and the wet breaking length and the dry breakinglength were determined. The results are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                       Dry breaking                                     Graft polymer Wet breaking length [m] length                                Exam- according to        aged       [m]                                        ple 11 Example unaged (5 min at 110° C.) unaged                      ______________________________________                                        a)    none       150        170      3309                                       b) Comparison 3 631 799 4013                                                  c) Comparison 2 822 1003  4389                                                d) 1 807 927 4823                                                             e) 2 771 837 4477                                                             f) 3 699 737 5122                                                             g) 4 819 910 4721                                                             h) 5 716 813                                                                ______________________________________                                    

We claim:
 1. A graft polymer of a polymer containing vinyl ester orvinyl alcohol units and an ethylenically unsaturated compound, whichgraft polymer is obtainable by free radical polymerization of(A)monomers or monomer mixtures comprising(a) from 1 to 100% by weight ofN-vinylformamide, (b) from 0 to 99% by weight of other monoethylenicallyunsaturated monomers copolymerizable with the monomers (a) and (c) from0 to 5% by weight of monomers having at least two ethylenicallyunsaturated, nonconjugated double bonds in the molecule,in the presenceof (B) polymers which contain units of vinyl esters of saturated C₁ -C₄-carboxylic acids or vinyl alcohol units, or of mixtures which containthese polymers and polymers having at least 3 alkylene oxide units orpolytetrahydrofuran, in a weight ratio (A):(B) of from 95:5 to 10:90 andsubsequent elimination of from 1 to 100% of the formyl groups of thegrafted-on N-vinylformamide by hydrolysis with formation of units of theformula ##STR20##
 2. A graft polymer as claimed in claim 1, whereinN-vinylformamide is used as monomer (A) and polyvinyl alcohols are usedas polymer (B), and from 1 to 100% of the formyl groups of thegrafted-on N-vinylformamide are eliminated from the graft polymer byhydrolysis with formation of units of the formula
 3. A graft polymer asclaimed in claim 1, wherein monomer mixtures of (a) from 1 to 99% byweight of N-vinylformamide and(b) from 99 to 1% by weight of vinylformate or vinyl acetate are used as (A) and polyvinyl alcohols are usedas polymer (B), and from 1 to 100% of the formyl groups of thegrafted-on N-vinylformamide are eliminated from the graft polymer byhydrolysis with formation of units of the formula ##STR21## and from 2to 100% of the formate or acetate groups of the grafted-on vinyl formateor vinyl acetate are eliminated with formation of units of the formula##STR22##
 4. A graft polymer as claimed in claim 1, wherein from 30 to95% of the formyl groups are eliminated from the graft polymer.
 5. Agraft polymer as claimed in claim 1, wherein from 30 to 95% of theformyl groups are eliminated from the graft polymer and from 30 to 95%of the formate or acetate groups are eliminated from the grafted-onmonomers.
 6. A process for the preparation of a graft polymer of apolymer containing vinyl ester or vinyl alcohol units and anethylenically unsaturated compound, comprising grafting by free radicalpolymerization (A) monomers or monomer mixtures comprising(a) from 1 to100% by weight of N-vinylformamide, (b) from 0 to 99% by weight of othermonoethylenically unsaturated monomers copolymerizable with the monomers(a) and (c) from 0 to 5% by weight of monomers having at least twoethylenically unsaturated, nonconjugated double bonds in themolecule,onto (B) polymers which contain units of vinyl esters ofsaturated C₁ -C₄ -carboxylic acids or vinyl alcohol units, or ofmixtures which contain these polymers and polymers having at least 3alkylene oxide units or polytetrahydrofuran, in a weight ratio (A):(B)of from 95:5 to 10:90, followed by eliminating by hydrolysis some or allof the formyl groups from the grafted-on N-vinylformamide of the graftpolymer with formation of units of the formula ##STR23##
 7. A processfor the preparation of graft polymers as claimed in claim 6, whereinmonomer mixtures of (a) from 1 to 99% by weight of N-vinylformamideand(b) from 99 to 1% by weight of vinyl formate or vinyl acetate areused as (A) and polyvinyl alcohols are used as polymer (B), and from 1to 100% of the formyl groups of the grafted-on N-vinylformamide areeliminated from the graft polymer with formation of units of the formula##STR24## and from 2 to 100% of the formate or acetate groups of thegrafted-on vinyl formate or vinyl acetate are eliminated with formationof units of the formula ##STR25##
 8. The graft polymer as claimed inclaim 2, wherein said polyvinyl alcohols are esterified to a degree ofup to 25 mol % with saturated C₁ -C₄ -carboxylic acids.
 9. The graftcopolymer as claimed in claim 3, wherein said polyvinyl alcohols areesterified to a degree of up to 25 mol % with saturated C₁ -C₄-carboxylic acids.
 10. The process of claim 7, wherein said polyvinylalcohols are esterified to a degree of up to 25 mol % with C₁ -C₄-carboxylic acids.